Modulation



Patented Oct. 12, 193.7-

i I i 2,095,311 PATENT OFFICE MODULATION Q Johannes Rohnfeld, Berlin, Germany, assignor to Telefunken Gesellschaft fiir Drahtlose Telegraphic m. b. H., Berlin, Germany, a corporation of Germany Application October 24, 1934, Serial No. 749,727

In Germany October 25, 1933 Claims. (crite ia) that thescreen-grid tube ofiers advantages over the triode. For instance, the .plate-to-grid capacitance in a screen grid tube is lower. One

chief. disadvantage residing in plate potential V modulation with a screen-grid tube consists in the risk of over-loading thescreen -grid whenever theplate potential is low. 7 The screen grid maybe protected by making the biasing voltage of the control grid from, the outset stronglynegative. When this isdone the power delivery of the tube decreases -'considerablyeven when the plate Voltageis high. H r l l Now} according to 'this invent-ion overloading of the screen g ridelectrode is avoided without sacrifice o'fthe power-output by insuring that the screen-gridvoltage, upon diminution of the plate voltage, is likewise reduced. In other words, screen grid potential and plate potential are modulated-under eo-phasal conditions." This method offers the further merit that a modulation characteristicthat is -straight"inside" wide limits is obtainable, thereby insuring linear modulation over a large potential-range.

' This fact will become clearer by reference'to Figs. l-8 of the drawings, of which Figs. 1', 3, 5, 7,

represent the relation between the plate potential Ea and the screen-grid potential E5, while Figs. 2, 4} {6, 8 illustrate the modulation characteristics, i. e., the relatio'n between the plate alternating V voltage e. and the plate direct current voltage 40 Ea, inthe presence of constant radio frequency control grid alternating voltage, which areinsured in the presenceof conditions shown in Figs} '1', '3;, 5 7, between the potentials Ea arising at the plate and Beat the screen grid. i

" Figures 9 to 13 inclusive illustrate various circult arrangements each including meansfor applying modulation potentials simultaneously to the anode and another electrode in a thermionic tube. l I V In the case of Fig. 1, conditions are so arranged that there prevails a linear relationship between applied platevoltage E5 and applied screen grid voltage'Es; In the examplehere chosen, the ratiofbetween the two is 25%, in other words, the screen-grid voltage amounts to only one-fourth 'of the-plate voltage. V The assumption is more,-

over made that both the modulation of the plate voltage as well as that of the screen-grid voltage is e. g. 0 to 400 volts and 0 to 100 volts, re-- spectively. The modulation. characteristic obtained from the .tube under these circumstances. is "shown in Fig, 2. In this graph, ea is radio frequency anode voltage plotted as ordinates 7 against anode voltage plotted as abscissa, it being understood that the relation between anode volt-' age Ea and screen grid voltage Es is maintained as shown by the graph in Fig. 1 while plotting the graph of Fig. 2. The characteristic is 7 straight only within the median ranges;

In Fig. 3 linear relations are shown between the plate potentialE'a and the screen-grid potential E5, but in this instance the plate potential is modulated 100% and the screen-grid voltage only 50%. But the relationship between the two voltages is riotconstant here. 'The modulation curve obtained under these conditions hasa shape asshown in Fig. 4. Note that modulating the screen grid between 50' and 100 volts while the anode is modulated between (land 400 volts has resulted in the lowerend of the graph of Fig. 2 being straightened as shown in the graph of Fig. 4. Thus, we are attaining a more desirable modulation characteristic.

In Fig. 5 the relations between plate voltage Ea and screen-grid voltage Es are again linear. While here the plate voltage is modulated 100% the screen-grid voltage is modulated Here,

bymdulat ingthe screen grid between 50 and 1Q0volt-s while the anode is modulated between 0 and 400 volts; I have straightened the upper end of the modulation characteristic graph, as shown in Fig. 6, but in doing so have made the lower endof athe graph less linear than in the prior cases.

g In the case of Fig. 7 the relations between plate voltage Ea and screen-grid voltage E's are not linear. The modulation 'voltage is impressed upon the screen grid by way of means possessing non-' linear action, the said means in the case of Fig.9 consisting of a triode tube. By producing a bilinear relation between the anode voltage and screening voltage,. as shown inFig. 'l, the ideal relationbetween the'anode radio frequency voltage and the anode direct current voltage "is obtainedfas shown in Fig. 8. The lower part of the graph of Fig. '7 is linear and rises steeply and then risesless steeply, but is still linear. This relation between the applied anode voltages and screen grid direct current voltages is accomplishedby producing a suitable distortion of the'screen grid'controlling voltage as it is applied.

To illustrate circuit schemes wherein modu- Iatloncharacteristics of the kind shown above may .be obtained, a number of exemplified embodiments are shown in Figs. 9-13. Referring to the same, It is the tube to be modulated, K is the direct current blocking condenser, D a radio frequency choke-coil, S the oscillation circuit,

and M the modulation transformer.

V The circuit organization of Fig. 9 serves for realizing the characteristics shown, in Figs. 1 and 2. The tube R has its control grid and cathode connected to a carrier wave source not shown. The anode of R. is coupled to the cathode of R by a blocking condenser K and a tuned circuit S. The latter is coupled to a load circuit which, for purposes of illustration is shown as an aerial system. Modulating potentials are applied from a source not shown to the primary winding of transformer M, the secondary winding of which is connected to a potentiometer P. One terminal of P is connected to the anode of R by way of a radio frequency choke D. A point on P is connected to the screen electrode of R. The other end of P is connected by the source of direct current potential to the cathode of R. The relation of four to one may be maintained between the anode voltage and screen grid voltage, as shown in Fig. 1, by adjusting the point on P, to get'the modulation characteristic as shown in Fig. 2. Operating states as shown in Figs. 3 and 5, and Figs. 4 and 6, respectively, are obtainable by circuit arrangements of the kind illustrated in Figs. 10-12. In Fig. 10 a special counterdirect current voltage source G is provided which serves the purpose of preventing the screen grid from obtaining a voltage equal to the plate voltage. In the case of Fig. 11 the alternating voltages to be fed to the screen grid are obtained by tapping the modulation transformer M. In both circuits the relation between the anode voltage Ea and screen voltage Es shown in Figs. 3 and may be obtained by the source G and movable points on P and the secondary of M, respectively, to obtain the modulation characteristic as shown in Figs. 4 and 6, respectively. In Fig. 12 an exemplified embodiment for a modulation transformer is shown in which the plate and the screen grid alternating potentials are generated in distinct secondary windings. By selecting the proper relation between the secondary windings of M and the primary winding and by moving the tap connected at the lower end of the secondary winding connected with the screen grid electrode along the anode source, the desired relation between the anode potential and screen grid potential may be obtained. Circuits I0, II, and i2 are in other respects quite similar to the circult of Fig. 9.

In a circuit scheme as shown in Fig. 13 it is feasible to insure working conditions of the kind depicted in Figs. land 8. In the screen-grid lead is included a non-linear device N which here, for instance, consists of a supplementary electron tube. To distort the screen-grid voltage, however, recourse may be had to any other known arrangement. The device N comprises a thermionic tube having its control grid connected to a point on the secondary winding of M and its anode connected to the screen grid of R. The cathode of the tube in N is connected to a point on a source of potential in series with a resistance connected between the screen grid of R and the lower end of the secondary winding of M. By properly biasing the control grid of the tube in N, the bi-linear relation between the anode voltage Ea and the screen grid voltage Es shown in Fig. '7 may be obtained. Note here that the first part of the graph is linear and rises steeply, as shown in Fig. 3, while the upper end of the graph is linear but rises less steeply, as shown in Fig. 4. The bilateral relation between the screen grid voltage and anode voltage combine to give a modulation characteristic linear throughout its length, as shown in Fig. 8.

I claim:-

1. A device for modulating carrier wave oscillations at signal frequency comprising, a thermionic tube having an anode, a cathode, and a control grid electrode, and a screen grid electrode, a carrier wave energizing circuit connected to the control grid and cathode of said tube, a source of modulating potentials, a reactance connected between a point on said source of modulating potentials and the anode of said tube, a connection between said source of modulating potentials and the cathode of said tube, and an impedance of non-linear character connected between the screen grid of said tube and said source of modulating potentials.

2. In a device for modulating carrier wave oscillations at signal frequency, a thermionic tube having an anode, a cathode and a control grid and a screen grid electrode, a carrier wave energizing circuit connected to the control grid and cathode of said tube, an impedance on which signaling potentials may be impressed, a circuit connecting said impedance between the anode and cathode of said tube, and a circuit including an impedance of non-linear character connecting a point on said first named impedance to the screen grid of said tube.

3. In a modulation system, a thermionic tube having an anode, a cathode, a control grid, and a screen grid, a circuit connected with the control grid and cathode of said tube for applying carrier wave oscillations to be modulated to the control grid and cathode of said tube, an impedance connected with a source of modulating potentials, a source of direct current potential connecting one terminal of said impedance to the cathode of said tube, a circuit connecting the other terminal of said impedance to the anode of said tube, and a circuit including a non-linear impedance connecting the screen grid of said tube to a point on said first impedance.

4. A system as recited in claim 3 in which a source of potential is interposed in said circuit connecting said screen grid to said point on said first impedance.

5. In a modulation system, a thermionic tube having an anode, a cathode and a control grid and a screen grid, a circuit for applying carrier wave oscillations to be modulated to the control grid of said tube, a modulation frequency transformer having a primary winding which may be connected to a source of modulating potentials, said transformer having a secondary winding, a source of direct current potentials connecting the secondary winding of said transformer between the anode and cathode of said tube, an additional thermionic tube having an anode, a

cathode and a control grid, a circuit connecting the anode-to-cathode impedance of said additional tube between the screen grid of said first named tube and a point on said secondary winding, and a connection between the control grid of said additional tube and a point on said secondary winding.

J OHANNES ROHNFELD. 

